In a working machine to which the hybrid system of this kind is applied, namely a so-called hybrid working machine, such technology is known that the power of an engine is assisted by powering operation of a motor generator using the electric power of an electrical storage device when the load of the hydraulic pump is large, and the electrical storage device is charged by regeneratively driving the motor generator by the engine.
Further, as one of the related art of the hybrid working machine, hybrid construction machinery is known which includes an engine control means that controls the power of an engine and a motor generation control means that controls the motion of a motor generator, in which the engine control means operates the engine with a droop characteristic in which the torque increases at a constant rate according to drop of the rotational speed, and the motor generation control means operates the motor generator with the rotational speed control which effects control so that the actual rotational speed agrees to a target rotational speed by giving the target rotational speed (refer to PATENT LITERATURE 1, for example).
In concrete terms, according to the hybrid construction machinery of this related art, it is configured that the motor generation control means gives a rotational speed corresponding to a region of low fuel efficiency and low exhaust in the operation characteristic (rotational speed-torque characteristic) of the engine to the motor generator as a target rotational speed, and operates mainly the motor generator by the rotational speed control. At this time, the engine generates the torque that corresponds to the controlled rotational speed according to the droop characteristic by the engine control means, and therefore, even when the load of the vehicle body may change, the rotational speed and the torque of the engine can be maintained constantly within a range of low fuel efficiency and low exhaust.